In a method for operating a digital device of this type, it has been practiced in the past that the digital device such as a camera is operated by a command based on an instruction packet from operation/control means such as a personal computer, or the digital device is triggered by the control from external control means.
As a conventional method for transmitting a digital data such as an image data, a data transmission method has been known, which can transmit the data efficiently according to an image data compression technique such as MPEG.
However, (1) in the conventional method for operating the digital device as described above, when the operation of the digital device is controlled by an instruction packet as described above, much time is required, e.g. a time to generate the instruction packet, a time to transmit the instruction packet, a time to interpret the instruction packet, and a time to execute the instruction. In this respect, there has been a problem in that the timing to operate the device cannot be accurately controlled by the instruction packet.
Also, when it is tried to trigger the digital device by the control from external control means, complicated wiring is required such as the wiring for a separate signal line.
(2) In the conventional transmission method for transmitting digital data as described above, when the data amount to be transmitted rapidly is increased, the data cannot be sent out within the anticipated time. In such case, particularly when a network is shared with other devices based on time division multiplex system, competition of the packet may occur if the sending of the data is not stopped within the scheduled time.
(3) When an image compression technique such as MPEG is used and the image data is compressed using an information of the changes between frames, a system is adopted to use an information of I frame (compressed and coded image within a frame) and to express the changes of a series of the subsequent image data, using the image data as a starting point.
However, according to this system, when multiplex transmission is performed for the image data from a plurality of image information devices such as camera on a single transmission route, the receiving of the image data from an image information device currently in transmission should be stopped and it should be switched over to the receiving of the image data from another image information device. When the receiving of the information of the switched image data is started from the middle of I frame, the image data cannot be constructed until the next I frame is received. Thus, the image is interrupted at the switching of the image information devices.
(4) Also, when a plurality of data transmitting devices such as photographing means and a plurality of data receiving devices such as monitoring means are connected over the network, and when transmission is requested individually from the data receiving device to the data transmitting device, it is necessary at first to inquire operating condition and channel of the data transmitting device. This results in the increase of traffic and leads to the increase of the processing amount of each device.
(5) When a data receiving device switches over the data transmitting device via a trunk line of the network, it has been practiced in the past that the data receiving device connected to the local bus transmits only the necessary data by connecting packet transmission control means between the local bus and the trunk line because data transmission in the local bus is not so rapid as in the trunk line of the network. Under the condition that the necessary data varies according to the time, and when data transmitting device is switched over by means such as instruction, there may occur error in the period from the sending of the switching instruction to the actual switching, and this means that it is not possible to control accurately.
(6) In the system to transmit the compressed data by controlling compression rate in such manner that the data amount averaged by time is on a constant level, there is no surplus time between the data groups to be transmitted. In this respect, when the data group is switched over to the next group, the compressed data after a certain data amount must be cut off.